Conventionally, it is known that non-linear signal distortion is generated in an analog circuit, an RF (Radio Frequency) circuit and the like making up a transmitting system such as a radio communication device. To compensate for such signal distortion, a technique called adaptive digital predistortion has been developed.
In this technique, the reverse characteristics of an analog circuit and an RF circuit are stored in a LUT (Look Up Table) in the form of compensation coefficients in accordance with the amplitude and the power of the input signal. With this configuration, distortion compensation is achieved by preliminarily applying to an input signal a compensation coefficient corresponding to the amplitude and the power.
In addition, in this technique, the input signal and the transmission signal to which the compensation coefficients have been applied are compared with each other, and the compensation coefficients are adaptively updated such that the difference between the input signal and the transmission signal is reduced. In this manner, even in the case where the distortion characteristics are changed under the influence of temperature change, voltage change and the like, compensation of signal distortion can be effectively executed.
For example, PTL 1 discloses a technique intended to compensate for signal distortion generated by an amplifier. In this technique, a compensation coefficient of compensation of signal distortion is generated by an adaptive algorithm based on a difference between an input signal and an output signal of the amplifier.
It is known that signal distortion has a memory effect. Memory effect is a phenomenon in which signal distortion is dependent not only on a current input signal, but also on historical input signals. In the technique disclosed in PTL 1, however, memory effect is not taken into consideration, and an effect of limiting signal distortion cannot be sufficiently obtained.
To solve such a problem, PTL 2 discloses a technique in which L×M compensation-coefficient candidates corresponding to possible L states of the power (or amplitude) of a current input signal and possible M states of the power (or amplitude) of historical input signals are preliminarily stored in a memory, and, from the candidates, one compensation coefficient corresponding to the power (or amplitude) of the current and historical input signals is read out, and then, the compensation coefficient is applied to the input signal.